Import setenv and unsetenv from gnulib

Import setenv and unsetenv from gnulib

This patch supersedes
https://sourceware.org/ml/gdb-patches/2017-07/msg00009.html

---

Patch [1] broke a build on MinGW hosts, because MinGW doesn't provide POSIX
functions setenv () and unsetenv ().  This can be fixed by using
implementations from gnulib.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=9a6c7d9c0

gdb/ChangeLog
yyyy-mm-dd  Anton Kolesov  <Anton.Kolesov@synopsys.com>

	* gnulib/update-gnulib.sh (IMPORTED_GNULIB_MODULES): Add setenv and
	unsetenv.
	* gnulib/aclocal.m4: Regenerate.
	* gnulib/config.in: Regenerate.
	* gnulib/configure: Regenerate.
	* gnulib/import/Makefile.am: Regenerate.
	* gnulib/import/Makefile.in: Regenerate.
	* gnulib/import/m4/gnulib-cache.m4: Regenerate.
	* gnulib/import/m4/gnulib-comp.m4: Regenerate.
	* gnulib/import/m4/environ.m4: New file.
	* gnulib/import/m4/setenv.m4: New file.
	* gnulib/import/setenv.c: New file.
	* gnulib/import/unsetenv.c: New file.